It is known in the art relating to push-pull MIG welding torches that the transferring of welding current from the MIG torch to aluminum welding wire is hindered by the oxide layer on the surface of the welding wire. Insufficient or impeded energy transfer may cause drastic arc fluctuations and corresponding welding defects such as improper penetration, leg contours, high spatter, and burn back of the contact tip.
Two known methods exist for improving the conductivity from the MIG torch to the welding wire. The first method is to enhance the contact between the contact tip and the welding wire by, for example, using a spring loaded mechanism. The second method is to apply a secondary current transfer point, i.e. a second current pickup point, while still maintaining the primary current transfer through the contact tip. For example, a bronze jump liner may be used inside the copper gooseneck, a metallic wire guide may be used next to the pulling drive rolls, or the drive gear may be made live. However, these methods have corresponding drawbacks, such as increasing the feeding forces in the case of a metallic jump liner, causing shavings of the welding wire as the welding wire rubs the metallic wire guide, and arc erosion of the drive gear if the drive gear is made live.
Alternatively, the idler wheel in a push-pull MIG torch can be made live. However, all of the idler wheels in existing push-pull torches are made of high carbon steel or stainless steel and use ball bearings to resist mechanical wear and to obtain low rotating frictions. The electrical resistance from the power cable to the surface of the idler wheel in these torches is typically 2 to 40 ohms. This is hundreds of times the total electrical resistance of a typical push-pull MIG torch, which is 5 to 10 milli-ohms.